Heated Mirrors

Hi all i just wondered if any of you guys/girls might be able to help me regarding my not so heated mirrors, I have a 2004 1.9tdi ghia the front and rear screen heat up but the mirrors do not, I have changed the fuse and even used a tester on the white/brown wires connected to the mirror and got no reading.
Is there a common fault with connections/wiring that i do not know about.
any help would be greatly appreciated
darryl